This invention relates to the injection lasers having active layers with a real refractive index waveguiding property due to a lateral spatial thickness variation (LSTV) profile in on active region of their active layers with fabrication of these profiles created by vapor phase epitaxy employing semiconductor substrates with a nonplanar pattern in their surface. The LSTV profile may be abrupt or gradual, or combination thereof, depending on the nonplanar pattern geometry and growth parameters. Specifically, this invention deals with the employment of nonplanar substrate patterns, such as, channels or mesas, of particular geometry that produce these profiles in the active layer of a multilayer semiconductor injection laser fabricated in metalorganic chemical vapor deposition (MO-CVD) or in molecular beam epitaxy (MBE), which have improved operating characteristics compared to previously known injection lasers with nonplanar active regions.
A class of lasers incorporating a nonplanar region in the active layer have been termed "nonplanar", denoting that the active layer possesses a nonplanar active region formed during laser fabrication. This nonplanar region is conventionally formed via liquid phase epitaxy (LPE) by growth of semiconductor layers over a semiconductor substrate having associated with its top surface a channel or a mesa. An example of the channelled substrate nonplanar laser (CSNP) is disclosed in U.S. Pat. No. 3,978,428 and in German OSL No. 2,716,750. The mesa substrate nonplanar laser (MSNP) is also disclosed in German OSL No. 2,716,750 and in the Article entitled, "High-Power Constricted Double-Heterojunction AlGaAs Diode Lasers for Optical Recording" by D. Botez, F. W. Spong and M. Ettenberg, Applied Physics Letters, Vol. 36(1), Jan. 1, 1980, pp. 4-6.
The strong interest in these types of lasers is attributed to their good operating characteristics, such as, low threshold current, linear light versus current characteristic, good temperature performance, and relatively symmetrical, anastigmatic output beam.
It has been established in research and development that semiconductor injection lasers having a nonplanar active region which includes a lateral spatial variation in the active region thickness that exhibits improved properties, such as, low threshold current, linear light output versus current characteristics and stable fundamental transverse mode control. These properties may be improved by selection of the proper spatial variation thickness and spatial variation rapidity. Nonplanar variations and rapidity are discussed in U.S. Patent Application Ser. No. 181,262 filed on Aug. 25, 1980, now U.S. Pat. No. 4,335,461, and entitled "Injection Lasers With Lateral Spatial Thickness Variations (LSTV) In the Active Layer", assigned to the assignee herein.
To date, nonplanar lasers have been successfully grown by LPE. Within the past few years, molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MO-CVD) have become important processes in the fabrication of single crystal semiconductor integrated optical devices, including injection lasers. These two procedures, to a large extent, are replacing the conventional LPE crystal growth techniques owing to their improved control over layer thickness, crystal composition, layer smoothness, abruptness of interfaces and uniform doping profiles.
LPE processes permit nonplanar variations in layer contours and thicknesses as desired. For example, LPE growth of channelled substrate lasers produced curved contours and thickness variations in deposited layers on the substrate. However, MBE and MO-CVD processes characteristically do not produce the same type of growth variations. Depending upon deposit rate, flow rate, substrate temperature, etc., the deposited layers or films tend to "match" the contour and shape of the depositing surface. It would be desirable to start with a substrate surface having a desired contour or taper adequate to replicate the desired variations in thickness and taper in sequentially grown layers. But, it is more practical to develop the desired taper and thickness variation during growth process, as done in the past with LPE relative to nonplanar substrate patterns, and obtain better accuracy and control in the desired contour and thickness variations that MBE and MO-CVD processes would provide.
One manner of providing desired lateral spatial thickness variations in the active region in MO-CVD processing is the employment of apertured masks (either temporary or integral) during the growth process, as disclosed in U.S. Patent Application Ser. No. 231,556, filed Feb. 4, 1981 and assigned to the assignee herein. The uniform depositing phenomena in MO-CVD is "interrupted" by the presence of the mask aperture producing a deposited layer through the mask aperture that varies laterally in spatial thickness.
Without the mask structure, the epitaxial deposition of layers will tend to take on the geometric characterization of the substrate surface. If a channel or mesa is present in or on the substrate surface, the subsequently deposited MO-CVD layers will tend to take on the nonplanar profile of the channel or mesa. In the case of flat channel bottoms or mesa tops, it has been previously observed that the subsequently grown MO-CVD layers including the active layer will have corresponding flat regions, replicating the form of the nonplanar profile. See the Article entitled, "Single-longitudinal-mode CW Room-temperature Ga.sub.1-x Al.sub.x As-GaAs Channel-guide Lasers Grown By Metalorganic Chemical Vapor Deposition", R. D. Dupuis and P. D. Dapkus, Applied Physics Letters, Vol. 33, page 724 et seq. (Oct. 15, 1978). While these regions are in a different plane relative to adjacent regions of the same layer, they have been thought to not exhibit spatial variations in thickness laterally along these flat regions. Such variations can be provided in MO-CVD processing by employing apertured masks, as previously mentioned.